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    Hey Ingo,
    So we've been seeing the following trace fairly frequently on our SMP
    boxes when running kernbench:

    BUG: at kernel/softirq.c:639 __tasklet_action()

    Call Trace:
    [<ffffffff8106d5da>] dump_trace+0xaa/0x32a
    [<ffffffff8106d89b>] show_trace+0x41/0x5c
    [<ffffffff8106d8cb>] dump_stack+0x15/0x17
    [<ffffffff81094a97>] __tasklet_action+0xdf/0x12e
    [<ffffffff81094f76>] tasklet_action+0x27/0x29
    [<ffffffff8109530a>] ksoftirqd+0x16c/0x271
    [<ffffffff81033d4d>] kthread+0xf5/0x128
    [<ffffffff8105ff68>] child_rip+0xa/0x12

    Paul also pointed this out awhile back:

    Anyway, I think I finally found the issue. Its a bit hard to explain,
    but the idea is while __tasklet_action is running the tasklet function
    on CPU1, if a call to tasklet_schedule() on CPU2 is made, and if right
    after we mark the TASKLET_STATE_SCHED bit we are preempted,
    __tasklet_action on CPU1 might be able to re-run the function, clear the
    bit and unlock the tasklet before CPU2 enters __tasklet_common_schedule.
    Once __tasklet_common_schedule locks the tasklet, we will add the
    tasklet to the list with the TASKLET_STATE_SCHED *unset*.

    I've verified this race occurs w/ a WARN_ON in

    This fix avoids this race by making sure *after* we've locked the
    tasklet that the STATE_SCHED bit is set before adding it to the list.

    Does it look ok to you?


    Signed-off-by: John Stultz <>

    Index: 2.6-rt/kernel/softirq.c
    --- 2.6-rt.orig/kernel/softirq.c 2007-06-05 18:30:54.000000000 -0700
    +++ 2.6-rt/kernel/softirq.c 2007-06-05 18:36:44.000000000 -0700
    @@ -544,10 +544,17 @@ static void inline
    __tasklet_common_schedule(struct tasklet_struct *t, struct tasklet_head *head, unsigned int nr)
    if (tasklet_trylock(t)) {
    - WARN_ON(t->next != NULL);
    - t->next = head->list;
    - head->list = t;
    - raise_softirq_irqoff(nr);
    + /* We may have been preempted before tasklet_trylock
    + * and __tasklet_action may have already run.
    + * So double check the sched bit while the takslet
    + * is locked before adding it to the list.
    + */
    + if (test_bit(TASKLET_STATE_SCHED, &t->state)) {
    + WARN_ON(t->next != NULL);
    + t->next = head->list;
    + head->list = t;
    + raise_softirq_irqoff(nr);
    + }

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      Last update: 2007-06-06 04:19    [W:0.042 / U:227.120 seconds]
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